Optimization of Electron Cyclotron Current Drive in the Magnetic Field Configuration of CHS

Second-harmonic electron cyclotron (EC) current drive experiments have been performed in the Compact Helical System (CHS). The driven current changes its direction according to the change of the EC-wave beam direction in agreement with an expectation from the Fisch and Boozer theory in the case of low-field-side injection of EC waves. The EC-driven current varies as a function of the magnetic axis position of CHS plasmas. The cause of the variation was experimentally investigated by a magnetic field scan. Setting the second-harmonic resonance layer near the magnetic axis was favorable to maximize the total EC-driven current. The main cause of the dependence of the driven current on the magnetic axis position is attributed to the change of distribution of the magnetic field along the beam path due to the change of the beam direction to aim at the magnetic axis in the three-dimensional helical magnetic field of the CHS.